docs: def join_in(self, *objs):
          """
          Create a join condition, connect A and C
          """
          if not objs:
              return self.table.c[self.fielda]!=self.table.c[self.fielda]
          else:
              keys = get_objs_columns(objs, self.reference_fieldname)
              return (self.table.c[self.fielda] == self.model_class.c[self.reversed_fieldname]) & (self.table.c[self.fieldb].in_(keys))